Benjamin Braddock arrives back in Los Angeles by plane, having just graduated from college and with his entire life ahead of him. He is smart, his family is wealthy and he has professional connections through his parents. Nevertheless, Benjamin is unhappy, overwhelmed by a sense of ennui and purposelessness. At home, his family throws him a graduation party, but Ben remains detached and expressionless. Congratulations are tossed his way, and unsolicited advice is administered freely. Overcome with a sense of existential claustrophobia and anxiety, Ben tries to get away, running inside and hiding behind the oversized aquarium in his room.
His bedroom door opens as the wife of his father's law partner, Mrs. Robinson, seems to have mistaken the room for the bathroom. Benjamin directs her to the bathroom, but Mrs. Robinson stays, smoking a cigarette and probing him about his apparent disillusionment, finally asking him if he would drive her home. When they arrive at her house, Benjamin reluctantly accepts her invitation to come in, where she pours drinks, puts on music, and engages in disarmingly candid conversation. It does not take long for Benjamin to realize that Mrs. Robinson is attempting to seduce him. Flatly denying that she is seducing him, Mrs. Robinson asks Benjamin if he’s ever seen the portrait of her daughter Elaine and takes him to Elaine's bedroom to show him. As he looks at the portrait, Mrs. Robinson starts to undress and asks Ben for help with the zipper. Feeling uncomfortable again, Benjamin tries to avoid helping her, but Mrs. Robinson once again assures him that she is not trying to seduce him. Before Benjamin can rush out of the house, Mrs. Robinson calls out for him to retrieve her purse and bring it upstairs. Mrs. Robinson is waiting for him upstairs, naked, and tells him that any time he wants to sleep with her, he can. The sound of a car outside sends Benjamin running past her and into Mr. Robinson downstairs. Mr. Robinson mixes Benjamin a drink and the two discuss Ben’s plans for his future, Mr. Robinson offering some fatherly advice that Ben should lighten up and fool around a little before finding one girl to settle with. As Benjamin leaves, Robinson reminds him that Elaine, their daughter, is coming back from college soon.
At another party thrown by his parents, Benjamin is turning 21 and his parents prepare their guests for the reveal of their big gift to Benjamin: a scuba diving outfit that he will model. Benjamin waddles towards the pool in the scuba suit, before jumping in and sinking to the bottom. In the next scene Benjamin calls Mrs. Robinson to invite her for drinks at the Taft Hotel. She agrees, and upon meeting him there pressures him to get a room for the night. Awkwardly, Ben makes a reservation under the name "Gladstone." After Mrs. Robinson and Benjamin go to the room, he hesitates to make the first move, but when Mrs. Robinson finally taunts him into action by slyly referencing his virginity, they begin a passionate affair.
Ben’s life for the following weeks swings back and forth between bored loafing at home and sleeping with Mrs. Robinson in the hotel room at the Taft. As Ben becomes lazier and lazier, his father questions his aimlessness, asking what he is doing with his life. Ben replies that he is drifting, and his father insists that a time comes in everybody’s life when they need to take stock and become proactive. The next time he’s with Mrs. Robinson in the hotel room, he asks if they can have a conversation rather than rushing in to sex. The conversation reveals that she and Mr. Robinson are no longer intimate, that she gave up a career in art for marriage, and that she only married him because she had gotten pregnant with Elaine. After agreeing to never tell Elaine Mrs. Robinson's secrets, Benjamin suggests that he might ask Elaine out on a date, but Mrs. Robinson expresses vehement opposition. Ben takes Mrs. Robinson's opposition to mean that she doesn’t think he’s good enough for her daughter and gets angry. Mrs. Robinson lures him back into bed by telling Benjamin that she was not implying that at all. After he promises Mrs. Robinson he will not, Benjamin's parents begin to pressure him to ask Elaine out on a date. He struggles to avoid the situation, but when his parents threaten that they will invite the entire Robinson family over for dinner instead, he chooses the lesser of two evils and asks Elaine out. On the night he picks up Elaine, he is met by a stone-faced Mrs. Robinson, smoking in the back room and expressing her disapproval. Ben tries to tell her he was pressured into it by his parents, but she remains cold and bitter.
The date does not go well. Benjamin tries to rush through the evening and behaves rudely to Elaine, speeding and taking her to a strip club. When Elaine starts crying and runs from the club, Benjamin tries to explain that he was pressured by his parents and that he is not behaving authentically. Spontaneously, he kisses her and they go eat dinner at a drive-in restaurant. Over hamburgers, Benjamin candidly shares his innermost thoughts about the unfairness of the world, which charms Elaine, as the two share a thoughtful and compatible relation to the world. After the two wind up at the Taft Hotel for a drink, Benjamin must fumble through an explanation as to why the entire staff is so friendly and calling him "Mr. Gladstone." Back in the car, Benjamin confesses that Elaine is the first person he can stand to be with for so long. After a pause, she directly asks if he is involved in an affair and he admits to having been in an affair, but remains vague about the details, lying that the woman has a son.
When Benjamin shows up for a second date the following day, Mrs. Robinson comes out to confront him, ordering him to never see her daughter again, and threatening that she will tell Elaine all about their affair. He responds by running toward the house and finding Elaine in her room. As he begins to tell the true story about the married woman, Mrs. Robinson appears behind Elaine and Ben freezes. The situation becomes clear to Elaine and she screams at him to leave. Benjamin spends the rest of his summer trying to get a glimpse of Elaine. After Elaine goes back to college, he announces to his parents that he is going to marry Elaine despite the fact that she doesn’t like him. Ben heads to Berkeley to find her. After taking a room at a boarding house, Benjamin continues spying on Elaine. He follows her bus ride to the zoo, where he meets her boyfriend, Carl, and tags along behind her until she finally demands to know what he’s doing, and tells him to leave her alone.
A few days later, Elaine shows up at Ben’s rooming house, demanding an explanation for the fact that he raped her mother. Benjamin tells her what really happened, which finally calms her down, and she leaves. After Elaine shows up at the rooming house again that night, Benjamin asks her to marry him, but she gives him a non-committal answer. For the next few days, Benjamin relentlessly pursues her, even after she tells him that Carl has also proposed. Later on, he buys an engagement ring and prepares for his marriage to Elaine, but when he returns to his room, Mr. Robinson is there waiting for him, and demands an explanation for the affair. He tells Ben he is divorcing his wife and when Benjamin tells him he is in love with Elaine, Mr. Robinson attacks him, promising him a life behind bars if he pursues his daughter.
When he tries to see Elaine again, Benjamin receives a note informing him that while she loves him, Elaine cannot marry him, given the circumstances. Ben determinedly goes to the Robinson house, climbs the fence and sneaks inside, where he finds Mrs. Robinson packing. She calls the police on him, but he makes a break for it, driving back to Berkeley only to find out that Carl and Elaine are getting married in Santa Barbara. Making a desperate drive to Santa Barbara, Benjamin attempts to get there before the wedding can take place. He discovers the location of the ceremony and as he gets closer, his car runs out of gas. Running the rest of the way to the church, Benjamin makes it to the balcony just as the ceremony is drawing to a close. He starts yelling and pounding on the glass, causing Mr. Robinson to start toward him.
In the midst of the confusion spreading throughout the congregation, Elaine turns toward the balcony and loudly yells Ben’s name, signaling her desire to escape her own wedding. Ben runs as fast as he can down to the ceremony and after Mrs. Robinson slaps her daughter twice, Elaine breaks free and runs out of the church with Benjamin. Benjamin grabs a cross off the wall and uses it as a wedge to keep the doors locked behind them.
The unlikely couple run toward a city bus, get inside, and take the bench seat across the back. Laughing and celebrating their successful escape, Elaine and Benjamin stare into the faces of the other passengers on the bus who have turned their heads to look at them.